1. Field of the Invention
The present invention relates to layered electrodes, and more particularly, to ultramicroelectrodes and various layered electrodes with thin film inorganic coatings. Chemical vapor deposition is used to apply an inorganic thin film coating to carbon fibers and foams, and metal wires, meshes and foams. The ultramicroelectrodes have highly reproducible electrode dimensions with improved sensitivity and electrochemical response.
2. Summary of Related Art
Electrodes are used for carrying or emitting electric charges (or electric charge carriers) and are often manufactured in a layered configuration. Layered electrodes are used in a wide variety of applications and in a number of different configurations. The chemical vapor deposition process for depositing thin layers of inorganic films provide a means for depositing insulating, semiconductor, metal, or superconductor film layers on extremely small conductors.
One of the primary applications for the present invention is for ultramicroelectrodes used in biosystems, microenvironments, gas phase detection, and ion-selective systems. Ultramicroelectrodes are made from very thin electric conductors with thicknesses (tip diameters) in the range of 0.5 to 50 micrometers. The ultramicroelectrodes are made from carbon fibers or metal wire of less than 10 micrometers. With the deposition of multiple layers of inorganic thin films in the present invention, the ultramicroelectrode provides an electrode with the desired structural integrity, performance characteristics and an overall diameter of less than 50 micrometers.
The majority of ultramicroelectrodes in use at this time are encased in a glass capillary. An insulating layer of glass with an epoxy resin seal is formed on the conductors to provide rigidity and structure to the conductors. Certain polymers have also been used to insulate the conductors of an ultramicroelectrode.
However, the use of the glass capillary system significantly increases the size of the ultramicroelectrode and results in a very fragile device. The size of the glass capillary (diameters of over 2 millimeters) makes the electrode too large for many applications.
Hairline fractures may occur in the glass because of the thermal stress caused by the difference of the thermal expansion coefficient in the glass and the conductor material. The cracks cause a changing surface area for obtaining electrode readings, which effects the reliability of the device.
The poor adhesion of the coating material to the conductor permits fluid to accumulate under the insulation. In addition, epoxy resins have poor stability with respect to organic solvents, which leads to the degradation of the epoxy insulation. These two problems could obviously effect the accuracy and reliability of the ultramicroelectrode in electrode measurements.
Because of the performance problems with glass and epoxy insulation, an increase in the residual current takes place. The signal/noise ratio is often unacceptable and the response time and reproducibility of the results are adversely affected. Many ultramicroelectrodes are still made by hand, which results in inefficient production operations, poor reproducibility, and non-competitive product costs.
Ultramicroelectrodes are used in applications based on both potentiometric and voltametric measurements. In general, only the tip of the ultramicroelectrode is exposed to the analyte solution. As the dimensions of the tip decrease, the quality of the voltammetric information increase and the ability to perform measurements in extremely small microenvironments or sub-microliter sample volumes is possible.
A major impetus for the development of voltammetric ultramicroelectrodes has been for in vivo measurements of electroactive species in biological microenvironments. Microelectrodes with diameters of 10 micrometers were used for electroanalytical probes of brain neurotransmitters. More recent efforts have focused on the development of ultramicroelectrode probes for measurements in single-cells with minimal perturbation of the cellular environment, which permits the investigation of chemical profiles at the cellular level. An improved ultramicroelectrode would benefit studies in biochemistry, cell biology, pharmacology, pharmacokinetics, and toxicology.
Ultramicroelectrodes will find widespread applications in electrochemical detection methods, including components of in vivo biosensors. Electrochemical methods are routinely used for analysis in a variety of clinical settings. Ultramicroelectrodes are being used in such processes as chromatography and capillary electrophoresis. Ultramicroelectrodes may also be used in such areas as neonatal units, where analysis can be performed without the need to continually draw blood samples from a premature baby with a limited blood supply.
Ultramicroelectrodes are also valuable for the investigation of interfacial phenomena within the diffusion layer of conventional electrodes. Since the diffusion layer typically extends less than a few hundred micrometers from the electrode surface, the small dimension of ultramicroelectrodes are well suited to achieve the spatial and temporal resolution necessary to observe changes in interfacial concentrations of the products and reactants of electrochemical reactions at larger electrodes.
Scanning electrochemical microscopy uses the small tip diameter of the ultramicroelectrodes to probe the character and reactivity of electrode surfaces with lateral resolution approaching the 100 Angstrom level.
As the microenvironment for analysis moves into cellular dimensions, or dimensions less than the thickness of the diffusion layer, one of the most important considerations is simply the physical size of the ultramicroelectrode probe. Carbon fiber or metal wire disk electrodes are constructed from fibers or wires with a diameter of 0.5 to 10 micrometers. If the insulation layer can be kept at a minimum thickness through the deposition of a thin film of an inorganic insulation layer, without compromising the performance of the ultramicroelectrode, improved measurements will be available from ultramicro-environments.
Electrodes fabricated by chemical vapor deposition techniques of inorganic thin films may be used in a number of other applications in addition to the ultramicroelectrodes for biosensors and other electrochemical detection applications. Metal mesh electrode applications include the use of thin meshes with thickness in the same micrometer range (less than 50 micrometers) deposited with one or more layers of inorganic thin film.
The metal mesh electrodes may be used as modified optical transparent electrodes, photo conducting arrays, acoustic conducting arrays, and conductor electrode arrays for such application as electrochromic display devices or solar cell collectors. In such electrochromic or solar cell devices, the inorganic thin film coating is a semiconductive, metallic, or superconductive thin film coating to enhance the charge carrying and/or charge storage capabilities of the mesh electrode.
Other applications utilizing electrodes made from the chemical vapor deposition process of the present invention include thin film coating of carbon foams and metal foams for use in electrocatalytic devices and flow through detectors. The electrodes may also be used for battery and photocell applications by combining, for example a plate conductor with an adjacent electroconductive ceramic plate.
The substrate materials and the layers of inorganic thin films may be selected from a variety of materials and may be applied in a number of different configurations. The inorganic thin film coating may be an insulating, semiconductive, metallic, or photosensitive material. In a number of different industries, there is a need for improved electrodes and smaller electrodes that are easy and cost effective to manufacture, and that provide the desired performance characteristics based on the materials and configuration of the electrode.
An ultramicroelectrode is disclosed in U.S. Pat. No. 4,959,130 (Josowicz et al.) utilizing a wire or filament of a noble metal or carbon on which polymer insulating layer is formed. The insulating layer is made from a compound consisting of substituted poly(1,4-phenylene) ethers, poly(1,4-phenylene) thioethers and poly(1,4-anilines), whose plurality of phenyl groups are cross-linked at their ortho-positions by alkylene groups with from two to ten carbon atoms.
U.S. Pat. No. 5,158,083 to Sacristan et al. discloses a miniature pCO.sub.2 probe. The probe includes a miniaturized glass bulb pH sensor arranged in a flexible hollow tube.
A dry glass electrode for use in potentiometric analyses of aqueous media is shown in U.S. Pat. No. 4,312,734 (Nichols). The conductor element is covered by an ion-selective glass, and the conductor element and the glass are heated to provide a seal and then cooled to anneal the glass.
U.S. Pat. No. 5,192,415 to Yoshioka et al. discloses a biosensor utilizing an electrode system made from conductive carbon paste.
U.S. Pat. No. 5,086,351 to Couput et al. shows an electrochromic element having an electrolyte ion conducting layer interposed between first and second inorganic electrochromic layers.
Chemical vapor deposition is known in a number of industries, and such a process has been used to deposit thin film coatings on glass and other substrates.
U.S. Pat. No. 3,808,035 to Stelter discloses a chemical vapor system for deposition of a single or multiple layers from a dilute gas sweep onto a variety of substrates.
A method for applying an inorganic coating to a glass surface was shown in U.S. Pat. No. 4,261,722 (Novak et al.). A gas stream containing a vapor of a metal compound which is thermally decomposable is directed onto a surface at an elevated temperature. The relative humidity is controlled to increase the rate of formation of the metal oxide on the surface.
U.S. Pat. No. 5,090,985 to Soubeyrand et al. discloses a method for preparing vaporized reactants consisting of a coating precursor and a blend gas for chemical vapor deposition.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention.
It would be desirable to be able to provide a low cost electrode, including an ultramicroelectrode and a metal mesh electrode, that is reproducible from both a dimensional and performance standpoint. Another desirable feature would be to provide a quality seal between the conductor and the insulator without epoxy or other sealant. In the biosensor applications, it would also be desirable to provide a ceramic insulation which is inert and biocompatible.